Aircraft on the ground need to be supplied with compressed air and electrical power. The usual source is an APU installed in the aircraft or where available, a ground cart. An alternative used in dual spool gas turbine turboprop engines is to run one engine while a propeller brake, connected to the reduction gear box (RGB), locks rotation of the low spool (i.e. the one that drives the propeller) while the high spool is permitted to run and therefore may supply compressed air to drive the generator. Turbofan engines, however, have neither a propeller brake nor an ROB, and thus cannot benefit from this solution. An improved solution more universally applicable to gas turbine engines is therefore desired.